Two factors that drive much of the development of electronic devices are the desire to increase the density of devices on a substrate and to increase the speed of such devices by reducing their response time. Both these factors are related to the overall performance of products that use electronic devices. An increase in the device density not only permits increased miniaturization of such products, but permits the deployment of a greater number of devices; this, in turn, permits greater versatility or functionality to be implemented with the devices. Increasing the speed of the individual devices also permits the functionality to be enhanced by permitting the execution of a greater number of instructions within any defined time period.
There are a number of approaches that have been investigated to improve both these factors. One technique for increasing the speed of devices may be understood with reference to FIG. 1, which provides a schematic illustration of the structure of a typical transistor 100. The device 100 includes source 104 and drain 108 regions within a semiconducting substrate 116. A material that is commonly used for the substrate 116 is silicon. Application of a voltage to a gate 112 permits current to flow between the source 104 and drain 108 through a junction 120. The speed of such a device may be increased by including and activating suitable dopants that act to increase the conductivity of the junction 120. For example, boron and arsenic are known to increase the conductivity of silicon when the structure is annealed to promote bonding of those dopants with neighboring silicon atoms. The annealing causes a rearrangement of the dopant electron structure that results in an improvement in conduction, and is sometimes referred to in the art as an “activation” step.
Activation is conventionally performed thermally by raising the temperature of the entire substrate. The effectiveness of the activation in promoting conduction is generally proportional to the temperature that is achieved, so that it is preferable to raise the temperature of the substrate to be close to its melting point. The melting point of silicon is 1410° C., so when activation is achieved with a thermal anneal, it is desirable to raise the temperature of the substrate to about 1300-1350° C. But at these temperatures, the diffusivity of the atoms in the substrate is also increased. While a thermal anneal may thus achieve the desired dopant activation, such an anneal also tends to raise the temperature of the substrate for relatively long periods of time. This reduces the level of control that may be exercised over the size and shape of the junction 120.
Both the ability to increase the conductivity of the junction and the ability to control its size and shape are desirable. There is accordingly a general need in the art for methods of activating dopants while retaining control over the size and shape of device junctions.